As an example of a starter having a planetary reduction gear device, a starter disclosed in JP-Y2-H6-23742 is known. As shown in FIG. 2, the starter has a planetary reduction gear device 120, a one-way clutch, an output shaft 150, and a pinion gear 160. The planetary reduction gear device 120 reduces the rotation speed of a motor 100 and transmits the rotation to a clutch outer portion 110 of the one-way clutch. The output shaft 150 slidably engages with an inner periphery of a clutch inner portion 130 of the one-way clutch through helical splines 140. The pinion gear 160 is provided on the end of the output shaft 150 on a side opposite to the motor 100. In starting an engine, the pinion gear 160 moves with the output shaft 150 to the left side in FIG. 2, thereby to engage with a ring gear of the engine.
In the starter, the output shaft 150 moves to the left side while rotating along the helical splines 140. When a stopper 170, which is formed on the output shaft 150, strikes a side face of a bearing 190 supported in a housing 180, the axial movement of the output shaft 150 is stopped. In this construction, after the stopper 170 strikes the bearing 190, a thrust force applied to the output shaft 150 through the helical splines 140 are likely to be largely applied to the bearing 190 and the housing 180. As a result, the bearing 190 and the housing 180 are likely to be damaged.
Further, by a reaction force caused when the stopper 170 strikes the bearing 190, the one-way clutch receives a thrust force in a direction of the motor 100, that is, to the right side in FIG. 2. This thrust force may result in defects to the operation of the one-way clutch and the reduction of the rotation.